ADM9240
SERIAL BUS INTERFACE
from the slave device. T ransitions on the data line must
occur during the low period of the clock signal and remain
stable during the high period, as a low-to-high transition
when the clock is high may be interpreted as a ST OP signal.
T he number of data bytes that can be transmitted over the
serial bus in a single READ or WRIT E operation is limited
only by what the master and slave devices can handle.
Control of the ADM9240 is carried out via the serial bus. T he
ADM9240 is connected to this bus as a slave device, under the
control of a master device, e.g., the PIIX4.
T he ADM9240 has a 7-bit serial bus address. When the device
is powered up, it will do so with a default serial bus address.
T he five MSBs of the address are set to 01011, the two LSBs
are determined by the logical states of Pin 1(NT EST _OUT /A0)
and Pin 2 (A1) at power-up. T hese pins have internal 75 kΩ
pull-down resistors, so if they are left open-circuit the default
address will be 0101100.
3. When all data bytes have been read or written, stop condi-
tions are established. In WRIT E mode, the master will pull
the data line high during the tenth clock pulse to assert a
ST OP condition. In READ mode, the master device will
override the acknowledge bit by pulling the data line high
during the low period before the ninth clock pulse. T his is
known as No Acknowledge. T he master will then take the
data line low during the low period before the tenth clock
pulse, then high during the tenth clock pulse to assert a
ST OP condition.
T he facility to make hardwired changes to A1 and A0 allows the
user to avoid conflicts with other devices sharing the same serial
bus, for example if more than one ADM9240 is used in a sys-
tem. Once the ADM9240 has been powered up, the five MSBs
of the serial bus address may be changed by writing a 7-bit word
to the serial Address Pointer Register (the hardwired values of
A0 and A1 cannot be overwritten). T hereafter, the new serial
bus address must be used to select the ADM9240, until it is
changed again, or the device is powered off.
Any number of bytes of data may be transferred over the serial
bus in one operation, but it is not possible to mix read and write
in one operation, because the type of operation is determined at
the beginning and cannot subsequently be changed without
starting a new operation.
T he serial bus protocol operates as follows:
1. T he master initiates data transfer by establishing a ST ART
condition, defined as a high-to-low transition on the serial
data line SDA while the serial clock line SCL remains high.
T his indicates that an address/data stream will follow. All
slave peripherals connected to the serial bus respond to the
ST ART condition, and shift in the next eight bits, consisting
of a 7-bit address (MSB first) plus an R/W bit, which deter-
mines the direction of the data transfer, i.e., whether data
will be written to or read from the slave device.
In the case of the ADM9240, write operations contain either
one or two bytes, and read operations contain one byte and
perform the following functions:
T o write data to one of the device data registers or read data
from it, the Address Pointer Register must be set so that the
correct data register is addressed, then data can be written into
that register or read from it. T he first byte of a write operation
always contains an address that is stored in the Address Pointer
Register. If data is to be written to the device, then the write
operation contains a second data byte that is written to the
register selected by the Address Pointer Register.
T he peripheral whose address corresponds to the transmitted
address responds by pulling the data line low during the low
period before the ninth clock pulse, known as the acknowl-
edge bit. All other devices on the bus now remain idle while
the selected device waits for data to be read from or written
to it. If the R/W bit is a 0, the master will write to the slave
device. If the R/W bit is a 1, the master will read from the
slave device.
T his is illustrated in Figure 2a. T he device address is sent over
the bus followed by R/W set to 0. T his is followed by two data
bytes. T he first data byte is the address of the internal data
register to be written to, which is stored in the Address Pointer
Register. T he second data byte is the data to be written to the
internal data register.
2. Data is sent over the serial bus in sequences of nine clock
pulses, eight bits of data followed by an acknowledge bit
1
9
9
1
SCL
D6
D2
0
1
0
1
1
A1
A0
R/W
D7
D5
D4
D3
D1
SDA
D0
START BY
MASTER
ACK. BY
ADM9240
ACK. BY
ADM9240
FRAME 1
SERIAL BUS ADDRESS BYTE
FRAME 2
ADDRESS POINTER REGISTER BYTE
1
9
SCL (CONTINUED)
SDA (CONTINUED)
D2
D5
D4
D3
D1
D0
D7
D6
ACK. BY STOP BY
ADM9240 MASTER
FRAME 3
DATA BYTE
Figure 2a. Writing a Register Address to the Address Pointer Register, then Writing Data to the Selected Register
REV. 0 –7–
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